1. Field of the Invention
The present invention is directed to methods, systems, and computer programs for removing an epithelial layer from a cornea of an eye. In particular, the present invention is directed at methods, systems, and computer programs for controlling exposure of the epithelial layer to ablative radiation in order to effect uniform and complete removal of the epithelial layer over a treatment region.
Ultraviolet and infrared laser based systems and methods are known for enabling ophthalmological surgery on the exposed surface of the cornea in order to correct vision defects. These procedures, generally referred to as photorefractive keratectomy, generally employ an ultraviolet or infrared laser to remove a microscopic layer of anterior stromal tissue from the cornea to alter its refractive power. In ultraviolet laser ablation procedures, the radiation ablates corneal tissue in a photodecomposition that does not cause thermal damage to adjacent and underlying tissue. Molecules at the irradiated surface are broken into smaller volatile fragments without heating the remaining substrate; the mechanism of the ablation is photochemical, i.e. the direct breaking of intermolecular bonds. The ablation removes stromal tissue to change the contour of the cornea for various purposes, such as correcting myopia, hyperopia, and astigmatism. Such systems and methods are disclosed in the following U.S. patents and patent applications, the disclosures of which are hereby incorporated by reference: U.S. Pat. No. 4,665,913 issued May 19, 1987 for "METHOD FOR OPHTHALMOLOGICAL SURGERY"; U.S. Pat. No. 4,669,466 issued Jun. 2, 1987 for "METHOD AND APPARATUS FOR ANALYSIS AND CORRECTION OF ABNORMAL REFRACTIVE ERRORS OF THE EYE"; U.S. Pat. No. 4,732,148 issued Mar. 22, 1988 for "METHOD FOR PERFORMING OPHTHALMIC LASER SURGERY"; U.S. Pat. No. 4,770,172 issued Sep. 13, 1988 for "METHOD OF LASER-SCULPTURE OF THE OPTICALLY USED PORTION OF THE CORNEA"; U.S. Pat. No. 4,773,414 issued Sep. 27, 1988 for "METHOD OF LASER-SCULPTURE OF THE OPTICALLY USED PORTION OF THE CORNEA"; U.S. patent application Ser. No. 109,812 filed Oct. 16, 1987 for "LASER SURGERY METHOD AND APPARATUS"; U.S. Patent No. 5,163,934 issued Nov. 17, 1992 for "PHOTOREFRACTIVE KERATECTOMY"; U.S. patent application Ser. No. 08/368,799, filed Jan. 4, 1995 for "METHOD AND APPARATUS FOR TEMPORAL AND SPATIAL BEAM INTEGRATION"; U.S. patent application Ser. No. 08/138,552, filed Oct. 15, 1993 for "METHOD AND APPARATUS FOR COMBINED CYLINDRICAL AND SPHERICAL EYE CORRECTIONS"; and U.S. patent application Ser. No. 08/058,599, filed May 7, 1993 for "METHOD AND SYSTEM FOR LASER TREATMENT OF REFRACTIVE ERRORS USING OFFSET IMAGING".
Of particular interest to the present invention, before the stromal tissue can be treated, an overlying layer of epithelial cells, referred to as the epithelial layer, is often removed. The epithelial layer is typically about 50 .mu.m thick, and removal has commonly been performed using a blunt spatula or other instrument for scraping the layer from the stroma.
Scraping the epithelial layer is disadvantageous in a number of respects. Use of a scraping instrument can impart irregularities to the stromal layer which can adversely affect subsequent treatment of the stroma using the laser radiation. Similarly, incomplete removal of the epithelial layer may also adversely affect subsequent reshaping of the stroma. Moreover, since the scraping is performed manually, it is usually necessary to remove more of the epithelium from an area larger than will actually be treated by the laser. The removal of extra epithelial tissue can increase the time necessary for healing. Additionally, use of a scraping instrument presents a small, but finite risk of infection to the patient.
For all of these reasons, the use of laser energy to remove the epithelial layer has been proposed. The use of laser energy avoids at least most of the complications associated with use of a scraping instrument, as described above. Moreover, use of the same laser which is used for subsequent treatment of the stroma would be particularly convenient and lead to a reduction in the overall time required for the treatment protocol.
Prior attempts to utilize single beam ablative radiation for removal of the epithelial layer of a cornea, however, have been limited by excessive removal of peripheral portions of the treated region, typically resulting in unintended removal of stromal tissue. Referring to FIGS. 1-3, laser systems 10 have been focused through apertures 12 so that a single beam of ablative radiation 14 is directed at the epithelial layer E disposed over a stromal layer S in a cornea C. It has been found that exposure of the epithelial layer E for a time sufficient to remove the full thickness of the layer at the center line CL of treatment (FIG. 2) results in excess removal of corneal material in the peripheral region P, as illustrated in FIG. 2. As can be seen, the epithelial layer E has been completely removed in a peripheral region P. In addition, a portion of the stromal layer S in peripheral region P has also been removed. Such reshaping of the exposed stromal surface prior to corrective treatment can adversely affect the subsequent treatment. While the initial, unintended reshaping of the stromal surface can be offset, for example, in the case of hyperopia, by increasing the diopter (flattening) of the subsequent laser treatment, the combined initial removal of the stromal material and subsequent correction of the initial removal results in a depression or well W being formed in the stromal layer, as illustrated in FIG. 3. While such an outcome has not been found to be clinically significant, it is undesirable to remove more stromal material than the amount which is necessary to effect the desired corneal reshaping.
For these reasons, it would be desirable to provide improved methods and systems for using ablative radiation to remove the epithelial layer from a cornea prior to photorefractive keratectomy to treat the underlying stromal layer. In particular, it would be desirable to provide methods and systems which remove the epithelial layer completely but without any significant removal of the underlying stromal layer. Such methods and systems should preferably utilize the same laser source and control systems employed in the subsequent photorefractive keratectomy treatment, should be simple to perform, and should overcome at least some of the deficiencies noted above.
2. Description of the Background Art
U.S. Pat. No. 5,505,724 describes use of a laser system for removing an epithelial layer prior to photorefractive keratectomy. While the '724 patent suggests that ablation of the epithelium in different areas of the treatment region should be the same, there is little description of how to achieve such a result.
U.S. Pat. No. 5,549,599 describes the use of a laser for removing epithelial and stromal tissue from an eye in successive steps. Switching from an epithelial removal mode to a stromal removal mode is performed manually or automatically based on a change in observed fluorescence. U.S. Pat. No. 5,634,920 describes the use of a laser for removing epithelial and stromal tissue from an eye. Epithelial removal is performed using a large beam until a change in fluorescence is observed. Different diameter beams are then defected at different remaining portions of the epithelium until it is entirely removed.